Deposition of metal films onto non-metallic inorganic (glass, silicon, quartz) and organic (polystyrene (PS), poly (methyl metha-crylate) (PMMA), polyethylene naphthalate (PEN), etc.) substrates is a common practice in industrial applications spanning from microelectronics to medicals (for example in dental care), artistic manufacture, for example, decorated porcelains and glasses, and many others. During fabrication processes, deposited metal films often undergo chemical and mechanical stresses. In particular, in the fabrication of microelectronic devices, both through the use of conventional techniques based on photolithography as well as through innovative printing methods, such as microcontact printing (μCP), deposited metal films may be subjected to wet or dry etching and/or peel-off treatments for creating a design functional patterning. Further details of microcontact printing may be disclosed in Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 2059-2067.
Should the deposited metal film not adhere properly to the substrate, these treatments may cause cracking and/or partial peeling of the patterned metal structure with a consequent reduction of mechanical and electrical properties. This problem is particularly acute when the deposited metal is gold, the adhesion of which to non-metal substrates is rather low. Enhancement of adhesion processes of deposited metal films is typically of paramount importance because it may significantly improve production yield on an industrial scale.
The poor adhesion properties of metals onto certain substrates may depend on specific interactions between metal and the substrate (chemical bonds, diffusivity), and on metal nucleation properties of the surface of the substrate receiving the deposited metal. For example, adhesion of metals onto oxide substrates (glass, silicon oxide, quartz), strongly depends on the affinity of the specific metal towards oxygen. In particular, the larger the heat of formation of the metal oxide, the greater is its adhesion to the oxide.
This explains, for instance, the poor adhesion of gold (formation heat of Au2O3=19 kcal/mole) compared to the very good adhesion of aluminum (formation heat of Al2O3=−399 kcal/mole). For this reason, in microelectronic fabrication processes, before depositing gold onto oxide substrates, it is usually preferred to interpose a metal layer having a good affinity with oxygen such as tantalum, aluminum, vanadium, chromium, titanium, tungsten or molybdenum as a bonding agent between gold and the substrate.
However, this method generally requires an additional metal deposition step, with a consequent increase of fabrication complexity and costs. Moreover, in fabrication processes that contemplate a selective etching of the deposited gold for patterning it, the under-laying adhesion layer may require a specific etching step not always compatible with the patterned gold portions. This is the case, for example, for the patterning of gold structures through microcontact printing of alkanethiols, wherein the etching of gold by alkaline solutions of iron cyanide would be ineffective with most metals that are normally used as an adhesion interlayer.
As an alternative, according to the prior art, adhesion of gold onto non-metallic substrates may be promoted with mercaptomethoxysilanes that are molecules capable of chemically bonding to both oxides and gold atoms. See for example, Celle, C.; Suspene, C.; Simonato, J.-P.; Lenfant, S.; Ternisien, M.; Vuillaume, D.; Organic Electronics, 2009, 10, 119-126.
However, this technique contemplates the use of organic solvents such as toluene, benzene, or other aromatic derivatives, and thus may hinder their use on plastic substrates. Plastic substrates may form the basis of important applications, of “Printed Electronics/Electronics on plastic foils.” That is they form the basis of all those printing techniques, including the microcontact printing technique for low cost applications.